This invention relates in general to hinges and more particularly to a switch hinge, that is, a hinge that is capable of making and breaking an electrical circuit.
The need to maintain surveillance over the entryways of buildings from central locations lead to the development of a wide variety of so-called switch hinges, that is, hinges having switches which will make or break an electrical circuit in response to the movement of the hinge leaves. One type of switch hinge has the switch, or at least the push button for the switch, exposed when the hinge is open. This invites intruders to tamper with the hinge and its switch and furthermore alerts intruders that the entry is under surveillance. The intruder will therefore seek some other entry into the building. A typical switch hinge of this variety appears in U.S. Pat. No. 3,715,537.
Another type of switch hinge contains a magnetic reed switch in one leaf and a magnet in the other leaf. When the leaves are brought together, the field of the magnet brings the reeds of the switch into contact and completes a circuit. A hinge of this variety must be made from a nonmagnetic material such as brass, and furthermore it is difficult to control the point at which the switch is actuated. U.S. Pat. Nos. 3,806,852 and 3,896,404 illustrate magnetic switch hinges.
Still another type of switch hinge has a button-type microswitch housed entirely within a knuckle for one of the hinge leaves. The switch is operated by the hinge pin which is fixed firmly in the knuckles of the other leaf, and normally some type of camming device is interposed between the push button of the switch and the hinge pin so that when the leaves move in relation to each other, the hinge pin depresses the switch button. A hinge of this variety is quite complicated and expensive to manufacture, and furthermore the switch and the camming device occupy portions of the knuckles which might otherwise be occupied by the hinge pin, thereby making the hinge somewhat weaker and less durable than hinges of conventional construction. Also, the hinge pin is normally secured in one of the knuckles by a set screw which is turned down against the side of the pin. While this provides a means for adjusting the angle at which the switch is actuated, it does not enable the adjustment to be performed with the precision that is required in many situations. In particular, the set screw, when turned down against the hinge pin, leaves a slight indentation or dimple in the pin. It is impossible to reset the screw a few degrees off of this dimple, for the screw will merely drop back down into the original dimple. Hinges of this variety appear in U.S. Pat. Nos. 4,066,857, 4,049,934 and 4,168,409.
Another switch hinge, which in effect houses the switch in a knuckles on one of the leaves, utilizes a slip ring assembly to make and break the electrical circuit, this being achieved by interrupting one of the slip rings so that when the contactor which normally bears against that slip ring comes to the dead spot, the circuit is interrupted. These switch hinges have generally the same deficiencies as switch hinges provided with microswitches in their knuckles. A slip ring-type switch hinge forms the subject matter of U.S. Pat. No. 3,838,234.
Yet another type of switch hinge has a spring loaded push rod that extends laterally through one of the leaves to the hinge pin and is moved axially by a camming surface on the hinge pin. The push rod in turn has a camming surface that bears against the actuator of a switch that is fitted to the leaf through which the rod extends. The hinge pin is secured to a knuckle of the other leaf by a set screw and as a consequence, it is difficult to adjust the angle at which the switch will operate with any precision. A switch hinge of this variety is illustrated in U.S. Pat. No. 3,840,715.